Line of Credit Facility (Details)	12 Months Ended
	Dec. 31, 2016 USD ($)
Line of Credit Facility [Line Items]
Revolving credit facility borrowing capacity	$ 650,000,000
Revolving credit facility weighted average outstanding balance	$ 70,139,000
Revolving credit facility weighted average interest rate, percent	1.40%
Option to increase facility size	$ 1,000,000,000
Line of credit payable	0
Line of credit facility available for future borrowings	$ 650,000,000
Line of Credit | LIBOR
Line of Credit Facility [Line Items]
Spread over LIBOR, percent	0.925%
Letter of Credit
Line of Credit Facility [Line Items]
Line of credit facility, undrawn letters of credit	$ 230,000